1. Field of Invention
The present invention relates to a method of monitoring chemical-mechanical polishing operation. More particularly, the present invention relates to a method of monitoring chemical-mechanical polishing operation using standard deviation of reflectance spectra as a monitored value.
2. Description of Related Art
As the level of integration of semiconductor devices increases, demand for precision finished products also soars. One of the major factors in determining the quality of devices is the degree of uniformity of a silicon wafer before photolithographic processing. Currently, chemical-mechanical polishing (CMP) is one of the most important processing steps for planarizing a silicon wafer in semiconductor production. In fact, chemical-mechanical polishing is capable of global surface uniformity. However, a large number of factors can affect the degree of uniformity in a CMP operation. One critical factor is the capacity to monitor the polishing end point in a polishing operation.
The dual damascene process is a commonly applied technique for fabricating highly integrated semiconductor circuits. FIG. 1 is a cross-sectional view showing a dual damascene structure formed by a conventional dual damascene process. First, as shown in FIG. 1, a metallic layer 12 such as aluminum or polysilicon is formed above a substrate 10, and then a dielectric layer 14 such as an oxide layer is deposited over the metallic layer 12. Thereafter, photolithographic and etching operations are conducted twice to form openings 18a, 18b and 20. The opening 18a acts as a via for coupling with the metallic layer 12, whereas a conductive material will be subsequently deposited into the openings 18b and 20 to serve as metallic interconnects.
Next, a barrier layer 22, for example, a titanium nitride/titanium (TiN/Ti) composite layer, is formed over the sidewalls and bottoms of the openings 18a, 18b and 20. Subsequently, metal such as tungsten is deposited to fill the openings 18a, 18b and 20 to form a metallic layer 24. Thereafter, using the barrier layer 22 and the dielectric layer 14 as a polishing stop layer, the metallic layer 24 is polished using a chemical-mechanical polishing method. Ultimately, a portion of the metallic layer 24 above the dielectric layer 14 is removed, forming a metallic plug. In the CMP operation, precise control of the polishing end point is a very important factor that deeply affects the quality of the surface finish. If polishing is stopped too early, metallic residue from the metallic layer 24 will remain above the dielectric layer 14, leading to possible bridging of neighboring circuits.
On the contrary, if the polishing operation is stopped too late, over-polishing of the metallic layer 24 will occur, leading to the formation of a concave surface (i.e., dishing of the surface as indicated by arrows 26 in FIG. 1).
In addition, in a dual damascene processing technique, over-polishing of the metallic plug will severely affect its sheet resistance. However, to ensure no residual metal will remain above the dielectric layer, some over-polishing is necessary. Therefore, for better monitoring of the polishing end point, one must rely on a highly reliable in situ end point detector (EPD). Note also that a conventional end point detector is capable of monitoring the polishing end point only. The end point detector is incapable of obtaining information such as the degree of uniformity of a polished wafer. Hence, if uniformity information is really needed, the wafer has to be inspected offsite with other instruments such as a profilometer or a microscope after the polishing operation has finished. Consequently, extra time is needed for inspection, and the information concerning the degree of uniformity cannot be immediately fed back to produce a precisely polished surface.
In light of the foregoing, there is a need for an improved method of monitoring the polishing end point and degree of uniformity while a chemical-mechanical polishing operation is being carried out.